Direct Observation of Aggregation-Induced Emission Mechanism

Article abstract of DOI:10.1002/anie.202004318

The mechanism of aggregation-induced emission, which overcomes the common aggregation-caused quenching problem in organic optoelectronics, is revealed by monitoring the real time structural evolution and dynamics of electronic excited state with frequency and polarization resolved ultrafast UV/IR spectroscopy and theoretical calculations. The formation of Woodward–Hoffmann cyclic intermediates upon ultraviolet excitation is observed in dilute solutions of tetraphenylethylene and its derivatives but not in their respective solid. The ultrafast cyclization provides an efficient nonradiative relaxation pathway through crossing a conical intersection. Without such a reaction mechanism, the electronic excitation is preserved in the molecular solids and the molecule fluoresces efficiently, aided by the very slow intermolecular charge and energy transfers due to the well separated molecular packing arrangement. The mechanisms can be general for tuning the properties of chromophores in different phases for various important applications.

Full text of DOI:10.1002/anie.202004318

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Direct observation of aggregation-induced emission mechanism
Authors: Jianxin Guan, Antonio Prlj, Rong Wei, Jie Peng, Kun-Han
Lin, Jitian Liu, Han Han, Zhihao Yu, Clémence Corminboeuf,
Dahui Zhao, and Junrong Zheng
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202004318
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10.1002/anie.202004318
Angewandte Chemie International Edition
Direct observation of aggregation-induced emission
mechanism
Jianxin Guan1†, Antonio Prlj2,3†, Rong Wei1†, Jie Peng1, Kun-Han Lin2, Jitian Liu1,
Han Han1, Zhihao Yu1, Clémence Corminboeuf 2*, Dahui Zhao1* and Junrong
Zheng1*
1College of Chemistry and Molecular Engineering, Beijing National Laboratory for
Molecular Sciences, Peking University, Beijing 100871, China
2Institut des sciences et ingenierie chimiques, École polytechnique fédérale de
Lausanne, CH-1015 Lausanne, Switzerland
3Department of Chemistry, Durham University, Durham DH1 3LE, United Kingdom
(Dedicated to the 100th Birthday of Professor Youqi Tang)
†These authors contributed equally to this work.
*Corresponding author. Email: junrong@pku.edu.cn (J.Z.); zhengjunrong@gmail.com;
dhzhao@pku.edu.cn (D.Z.); Clémence.Corminboeuf@epfl.ch (C.C.)
1
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Abstract
The mechanism of aggregation-induced emission, which overcomes the common
aggregation-caused quenching problem in organic optoelectronics, is revealed by
monitoring the real time structural evolution and dynamics of electronic excited state
with frequency and polarization resolved ultrafast ultraviolet/infrared spectroscopy and
theoretical calculations. The formation of Woodward-Hoffmann cyclic intermediates
upon ultraviolet excitation is observed within picoseconds in dilute solutions of AIE
molecule tetraphenylethylene and its derivatives but not in their respective solids. The
ultrafast cyclization because of the excited state crossing a conical intersection provides
an efficient nonradiative relaxation pathway in solutions. Without such a reaction
mechanism, the electronic excitation is preserved in the molecular solids and the
molecule fluoresces efficiently, aided by the very slow intermolecular charge and
energy transfers due to the well separated molecular packing arrangement in solids. The
mechanisms can be general for tuning the properties of chromophores in different
phases for various important applications.
Introduction
In nature, most luminescent molecules glow strongly in dilute solutions but dimly
in aggregate states. This phenomenon is well known as “aggregation-caused quenching
(ACQ)”[1]. In many practical applications, e.g. organic or polymer light-emitting diodes,
ACQ is a major limiting factor.[2] In sharp contrast, a small group of molecules behave
completely oppositely. The more they aggregate, the stronger their luminescence.[3] The
2
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aggregation-induced emission (AIE) phenomenon, in particular, of those molecules
packed with bulky aromatic rings[3c, 4] owing to their structural similarity to many
important optoelectronic molecules, has been considered as an intriguing direction to
overcome the problem of ACQ, which may open new doors for organic molecules in
optoelectronic applications.[5]
Puzzlingly interesting and seemingly promising in practice, AIE and
solidification-induced emission (SIE) drawing tremendous research interest from both
experiments and theory in the past 19 years,[5-6] have a history of over 100 years[3c, 7]
which is, unluckily, not lengthy enough to manifest their mechanisms. Long before the
AIE concept was booming in 2000s, the nonradiative decay of excited
tetraphenylethylene (TPE), a classical textbook AIE and SIE molecule, and its
derivatives in solution was portrayed in terms of E-Z photoisomerization,[8] borrowing
concepts, e.g. sudden polarization, phantom state etc., from the excited state dynamics
of well-studied ethylene and stilbene molecules. The importance of torsional motion[9]
was also pointed out, for instance the systematic studies of tethered TPEs suggested
that the phenyl ring torsions play a major role in deactivation of excited singlet state,
as opposed to traditionally assumed “olephinic twist” around the central double bond.
[10]
With the rise of AIE field about twenty years ago, for typical AIE and SIE
molecules with bulky aromatic rings, e.g. TPE alike, it has been generally proposed that
the nonradiative decay in solution occurs due to the unhindered intramolecular rotations
and vibrations, which serve as the energy acceptors for the electronic excitation
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energy.[1] Restriction of intramolecular rotations (RIR)[11] and vibrations (RIV)[12] in
aggregates, leading to the disappearance of these energy acceptors[1] so that the
electronic excitation energy is preserved, are responsible for AIE. The possibility of E-
Z photoisomerization in solution has been considered as well, but its significance was
ruled out.[13]
However, the situation has changed over the last several years. There has been a
series of experimental studies suggesting that the E-Z photoisomerization channel is the
dominant deactivation channel in solution-based TPE[6b, 6c, 14], Nevertheless, the direct
observation of E-Z isomerization in solution is deemed as experimentally difficult.[4b]
As for the solid state and aggregate, induced emission can be explained by the steric
restriction of isomerization process.[14a] Thus, the latest resolution of the dilemma
between intramolecular phenyl rotation and E-Z isomerization, which was recently
reviewed[4b], seems to favor the latter mechanism.
Despite the seemingly consensus being achieved, there has been a “third”
mechanism which has been ignored in most of the literature, namely the
photocyclization.[15] Theoretical studies have proposed[6n] that, after photoexcitation the
structure of electronic excited state of the molecule evolves and reaches a special point
at which the electronic excited state and an electronic ground state with highly excited
vibrations are degenerate. At the geometry of conical intersection (CI)[16], i.e. the
degeneracy between electronic states of the same multiplicity, the electronic excite state
turns into the electronic ground state resonantly, so no emission occurs. Following the
conversion, the electronic energy becomes mostly vibrational energy, and the highly
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excited vibrations of the electronic ground state rapidly dissipate their energy to other
intra- and inter- molecular vibrations and rotations within hundreds of femtoseconds to
picoseconds.[17] In case of TPE, upon passing the CI molecule converts to a Woodward-
Hoffmann type cyclic intermediate (or alternatively deactivates back to the original
ground state structure, e.g. via photoisomerization pathway). During this process,
although the unhindered intramolecular rotations and vibrations are necessary for the
fast nonradiative decay, they are not the energy acceptors. Instead, they provide the
degrees of freedom for the electronic-excited molecule to isomerize to a particular
structure. Upon aggregation, because of the imposed spatial and motional restrictions,
the energy barrier to reach the conical intersection is too high for this nonradiative
decay pathway to beat the emission channel. The electronic excitation is preserved and
the molecule fluoresces. However, this mechanism is plagued, as the real-time
structures evolving from the electronic excited state to the ground state in the dilute
solution and the aggregate state are still elusive from experimental observations.
Results and Discussion
Real time observations of excited state structural evolution and dynamics via
ultrafast UV/IR spectroscopy
In this work, combining molecular designs, theoretical calculations, and frequency
and polarization resolved ultraviolet/infrared (UV/IR) mixed frequency ultrafast
spectroscopy, we are able to observe the structural evolution crossing the conical
intersection in AIE molecules in real time, shedding light to the origins of AIE
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phenomenon in TPE-type molecules.
Fig.1. Structure of a typical AIE molecule tetraphenylethylene (TPE) and its cyclized
isomer (diphenyl dihydrophenanthrene) as well as ultrafast UV/IR spectra in the TPE
solid state. (a) Molecular structure of TPE and DPDHP. (b) Ultrafast UV/IR spectra at
waiting times of 0.1 ps, 0.5 ps, and 1 ps, respectively. O.D., Optical Density. They are
not normalized with the excitation intensity. (c) Spectra with excitation at 290 nm and
330 nm, respectively. The data are collected with the magic angle configuration.
Figure 1 displays the molecular structures of a typical AIE molecule
tetraphenylethylene (TPE) and its cyclic isomer diphenyl dihydrophenanthrene
(DPDHP) and the ultrafast UV/IR spectra at different waiting times of TPE solid. In
fig.1b, the x-axis is the excitation wavelength in the UV region from 280 to 360 nm and
the y-axis is the detection frequency in the IR range from 2900 to 3150 cm-1. Upon UV
radiation, the TPE molecule is excited to the electronic excited state and its electronic
and nuclear motions begin to evolve. The electronic excitation and the evolution of
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molecular motions lead to the appearance of vibrational peaks. Comparing spectra with
different excitation wavelengths, e.g. 290 nm and 330 nm in fig.1c, we notice that,
within experimental uncertainty, varying the excitation wavelength changes the
intensity of the signal, but barely changes the major feature C-H stretch frequencies
between 3000 cm-1 and 3100 cm-1. Therefore, in the following, only signal with
excitation at 290 nm which is close to the UV absorption center (fig.S1 in SI) is
presented. More discussions about the signal/noise ratio and excitation wavelength
independence are provided in SI (fig.S14&15&16).
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Cyclization process in TPE solution
Fig.2. Dynamics and energy profile of TPE after 290 nm excitation. Evolution of
vibrational spectra of TPE (a) in the solid state and (b) in a dilute (50 mM) THF solution
after excited with photons at 290 nm. Signal decays of (c) the solid sample and (d) the
THF solution detected at different IR frequencies. The signals reach maxima at time
zero in the solid sample, whereas much slower at about 10 ps in the THF solution (the
insert of (d)). The peaks at 1322, 1345, and 1569 cm-1 are reported in fig s5. (e) FTIR
spectra of TPE in the solid state (black) and the THF solution (red). The frequencies of
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the two major peaks are very similar, in contrast to the huge difference observed in the
transient spectra in (a) and (b). The major feature of the transient spectra of the solid
sample in (a) resembles that of FTIR spectra, distinctly different from that of the
solution in (b). (f) Computed energy profile of TPE after electronic excitation. The
excited TPE molecule can deactivate by the Woodward-Hoffmann excited state
cyclization (to the right), or by the cis-trans isomerization by twisting around the central
ethylene bond (to the left). (a) to (b) are rotation-free data (푃(푡) = (푃 (푡) +
∥
2 푃⊥ (푡)) ⁄ 3). Data measured with both parallel and perpendicular polarizations are
presented in fig.s3-s6 in SI.
Figure 2a&b display the evolution of vibrational spectra of TPE in the solid state
and in a dilute THF solution after excited with light of 290 nm. In the solid sample, two
absorption peaks at 1486 cm-1 and 1437 cm-1 appear upon UV excitation (fig.2a). The
peaks reach maxima almost instantaneously within the temporal resolution (about 100
femtoseconds) and diminish quickly. After 10 ps, their intensities have dropped by
more than 70% (fig.2c). These two peaks slightly redshift, compared to their respective
peaks at 1489 cm-1 and 1442 cm-1 in the FTIR spectrum (fig.2e), resulting from the
excited state absorptions of the benzene skeleton vibration modes and frequency shifts
induced by the electronic excitation.[18] Accordingly, two corresponding small
bleaching peaks grow in at their respective 0-1 transition frequencies, appearing at
slightly blue-shifted 1497 cm-1 and 1447 cm-1 because of peak overlaps. The positive
background of the signals in fig.2a is probably caused by the absorption of weakly
bound carriers generated by the UV excitation. Similar observations have been reported
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for organic materials.[19]
In the dilute solution, despite its FTIR spectrum is very similar to solid (fig.2e), its
ultrafast spectral evolution is distinctively different from the solid (fig.2b). Upon UV
excitation, a new absorption peaks at 1469 cm-1 (fig. 2b) grow in, as well as peaks at
1322, 1345, and 1569 cm-1 in fig. s5, and at 3031 cm-1, 3058 cm-1 and 3086 cm-1 in fig.
s7 in SI. It reaches maximum at around 10 ps (fig.2b&d). Signals at early waiting times
contain contributions from the solvent nonresonant response which decays much faster
than the TPE signal, see fig. s8. In general, three possible mechanisms can lead to the
appearances of these new peaks: the electronic excitation induced frequency shifts and
vibrational excitations (coherent coupling), the energy conversion and transfer because
of electron/vibration coupling, and the formation of new molecular structures initiated
by photoexcitation, e.g. ultrafast deactivation through the CI crossing.[20] Since the
signals of vibrational excitations or frequency shifts caused by electronic excitation are
required to reach maxima upon excitation by the Frank-Condon principle[21] as those in
the solid sample, they are not the cause of these new peaks of which the maximum
arrives several picoseconds after excitation. The energy conversion and transfer
because of electron/vibration coupling is not the reason either, because none of the
vibrational modes of TPE has the same frequencies as these new peaks in the transient
spectra. Therefore, the appearances of these peaks indicate the formation of a new
molecular structure. In other words, the system in solution crosses a conical intersection
and reaches a new electronic ground state. In fact, these peaks are the characteristic
vibrational signals of the long-anticipated new ground state, the theoretically predicted
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cyclic intermediate (fig.2f, fig.s5, s7 & s10 and table s1 in SI).
Our theoretical studies provide an interesting hint to possible pathways of the
dynamics. Upon photoexcitation, the central C=C double bond reduces its bond order,
and akin to the photoexcited ethylene molecule acquires a charge-resonance (S1 min)
character.[8d] The excited state dynamics follows twisting motion around the central
bond, until the conical intersection with a ground state is met. The cis-trans
isomerization along the central C=C can occur, but due to the symmetric structure of
TPE, the original ground state structure is restored after the isomerization. However,
due to the presence of conjugated phenyl substituents, another competing mechanism
of excited state deactivation may play a key role. The Woodward-Hoffmann excited
state cyclization involving six carbon atoms may proceed, forming a covalent bond
between the carbons of two neighboring phenyl rings. Figure 2f displays the computed
excited state energy profile of TPE. Comparing the two possible pathways, the
photocyclization appears as a barrierless process which requires a smaller
reorganization of nuclear degrees of freedom (yellow line in fig.2f from S0 min to the
yellow/blue crossing point on the right). While the isomerization seems energetically
more favorable, it requires large changes in nuclear coordinates, including the
relaxation to the shallow S1 minimum and crossing a small barrier to reach the conical
intersection of twist configuration (yellow line in fig.2f from S0 min to the left).
Although it is likely that both mechanisms coexist together, our simulations[15a, 15b]
showed that the cyclization process dominates. This prediction is confirmed by the
experimental results in fig.2b. As discussed above, at very early waiting times before
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any dynamic event occurs, e.g. 0ps, the absorption peaks in fig.2b are from the
photoexcitation-induced frequency shifts and therefore their intensities can be used to
represent the number of photo-excited molecules. In fig.2b, the absorption peak at 1435
cm-1 at early waiting times, similar to that at 1437 cm-1 in fig.2a of the solid sample, is
caused by the photoexcitation-induced frequency shift of the benzene skeleton vibration
mode. At 0.1ps, the intensity of this peak with background subtraction is similar to that
at 1469 cm-1, indicating that a substantial amount of photo-excited molecules have
converted into the cyclic structure. At 10ps, the peak at 1469 cm-1 is significantly larger
than that at 1435 cm-1. If the vibrational transition dipole moment of peak at 1469 cm-1
is assumed to be similar to that of peak at 1490 cm-1 of TPE, the sum of the two peaks
at 1435 cm-1 and 1469 cm-1 at 0.1ps normalized with the square of transition dipole
moment ratio (roughly 1:2) can approximately represent the number of photo-excited
molecules when most of them have not been deactivated. The percentage of initially
photo-excited molecules converted into the cyclic structure at 10ps can then be
estimated by the normalized ratio between the peak at 1469 cm-1 and this sum. The
result show that over 80% of the excited molecules has been converted into the cyclic
intermediate within 10ps. This estimation is approximate, however, as spectral overlaps,
the noise level (0.00005~0.0002), and the uncertain actual values of transition dipole
moments can add uncertainties to the result. Nevertheless, the semi-quantitative
evaluation suggests that a substantial amount (probably over 50%) of the photo-excited
molecules have become the cyclic intermediate within 10ps. A similar conclusion can
be drawn from analyzing data of a TPE CH2Cl2 solution (fig.s7 in SI) that has a different
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solvent background at around 1450 cm-1.
In our calculations, the cyclization process is barrierless for isolated molecules.
However, in solutions, the formation of cyclic intermediate involves many nuclear
motions of the bulky rings which requires the reorganization of solvation structures.
Therefore, its completion takes time and is dependent on solvation effects. As displayed
in fig.2d, it takes about 10 ps in THF for signal to reach maximum, and about 7~8ps in
CCl4 (fig.s7). The cyclic intermediate is significantly less stable than the TPE electronic
ground state (fig.2f). Under ambient condition, it can transform back into TPE, upon
photoexcitation or thermal molecular collisions. As shown in fig.2d, most of the cyclic
intermediates last for only about 300 ps before they transform back to TPE. However,
the lifetime is still sufficiently long for it to react with O2 in the solution to form cyclic
product, which is isolated with HPLC and detected by MS (fig.s9). The cyclization
observed in TPE follows the Woodward-Hoffmann rules that predict a cyclization of
excited 6-π-system that occurs via co-rotative torsion of conjugated moieties. The
results are consistent with various previous experiments. For example, photocyclization
has been reported experimentally as the first step in photo-oxidation of TPE to
diphenyl-phenanthrene.[22] Cyclic intermediate is known to be unstable, and if not
trapped reverts back to the original compound.[22c] Cis-stilbene (with two benzene rings
of TPE replaced by two Hs) is known to undergo photocyclization in solution,[23]
though trans-stilbene is a prototype system for E-Z photoisomerization. It was
estimated that up to 30% of the excited state decay of Cis-stilbene occurs via cyclization
channel,[23b]
which was well reproduced by the mixed quantum-classical
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simulations[24] similar to those performed for TPE. It is reasonable that TPE, having
two pairs of cis-phenyls, may undergo even more cyclization. Conversely, ethylenic
twist with more bulky groups may be less favorable.
The different effects of UV excitation on the TPE dilute solution and solid sample
can be understood from the standpoint of energy. In the dilute solution, the energy cost
for the solvent molecules to adjust their configurations to accommodate the rotations
and twisting of the benzene rings during the transformation of TPE to the cyclic
intermediate is essentially the THF solvent molecule interaction energy, which is less
than 1 kcal/mol.[25] In the solid sample, the rotations and twisting of the benzene rings
would require to break the crystalline structure. The energy cost is too high for it to be
feasible.
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The formation of cyclic intermediate in other AIE molecular solutions
Fig.3. Transient vibration spectra and kinetics of molecule 2 and 3 after UV excitation.
(a) ~ (d) Transient vibrational spectra of molecule 2 (a) and 3 (b) in dilute THF solutions
and in the solid state (c & d) after excited with light of 290 nm. (e) ~ (h). Signal decays
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of molecule 2 and 3 in THF solutions (e & f) and in the solid state (g & h). Data are
rotation-free.
Forming a cyclic intermediate is also observed in the other two solutions of AIE
molecules 2 and 3. The structures of these two molecules are displayed in the inserts of
fig.3a&b. They also have four bulky groups connected to a C=C double bond. Once
they are excited with UV light, they can transform into their respective cyclic
intermediates. Their spectral evolutions after excited with 290 nm light are shown in
fig.3a&b. Similar to TPE, new vibrational absorption peaks appear at different
frequencies which match reasonably well those theoretically predicted values of the
cyclic intermediates (fig.s10 and table s1 in SI). It is interesting that the time to reach
signal maximum is different for different molecules. It takes molecule 2 about 3.5 ps
and molecule 3 only 0.1 ps (fig.3e&f).The bulkier the side groups are, the faster the
formation of cyclic intermediate, as the initial configuration of the molecule with
bulkier side groups is closer to that of the cyclic intermediate and larger side groups
can hinder the competing twisting motion around the broken C=C bond. Similar to TPE,
in the solid samples of these two molecules, the photoexcitation does not generate any
new structure either. As displayed in fig.3c&d, only absorption and bleaching peaks
caused by photo-excitation-induced vibrational excitations and frequency shifts appear.
Slow process of charge/energy transfers in AIE molecular solids
Lack of accessible conical intersection allows solids to preserve much more
electronic energy needed for luminescence than is the case in the dilute solutions.
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However, that is not sufficient for the AIE solids to avoid the quenching problem caused
by charge or energy transfer that occurs in many other organic chromophores. There is
another important feature of AIE molecules that allows them to significantly alleviate
this problem and have relatively high luminescence quantum yields. The non-coplane-
oriented bulky side groups of AIE molecules prevent the excited electrons of one
molecule from staying close to another molecule in the solid. Because of this repelling
molecular stacking, the charge or energy transfer between molecules in closely-packed
solids that usually cause quenching are difficult, in other words, very slow to proceed.
The blocking of charge or energy transfer by molecular packing can be manifested with
time dependent anisotropy measurements (fig.4). Similar to energy transfer induced
anisotropy decay experiments[26], the anisotropy change in the present experiments
reflects the charge or energy transfer in the solids. The initial anisotropy of our
measurements is determined by the cross angle between the electronic transition and
vibrational transition dipole moments within the same molecule. Following
photoexcitation, if energy or charge transfers from the excited molecule to an adjacent
molecule which typically has a different orientation, the anisotropy of the signal will
change because after transfer it is determined by the cross angle between the electronic
transition dipole moment of the donor molecule and the electronic/vibrational transition
dipole moment of the acceptor molecule. As displayed in fig.4, the anisotropy values
of the ultrafast signals of AIE solids TPE, molecule 2, and molecule 3 do not change
up to 20 ps, unlike those of similar molecules with coplane-oriented side groups,
molecule 4 and 5, for which the luminescence is quenched upon aggregation within a
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few ps. The results indicate that charge or energy transfer is very slow in the AIE solids,
and the transfer occurs quickly in molecule 4 and molecule 5 where luminescence
centers stack very close to each other.
Fig.4. Time dependent anisotropy of UV/IR measurements. The Anisotropy value does
not change for AIE molecules (a~c) whereas it changes within a few ps for ACQ
molecules (d&e).
Conclusion
In summary, photocyclization by crossing the conical intersection, a process which
provides an efficient nonradiative relaxation pathway for electronic excitation and is
too fast for conventional experimental tools, is observed in dilute solutions of AIE
molecules TPE and its derivatives with ultrafast UV/IR spectroscopy. The
inaccessibility of such crossings in solids helps to preserve electronic energy for
luminescence. Large separation of luminescence centers among molecules in solids due
to the non-coplanar packing effect of the bulky side groups significantly slows
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intermolecular charge and energy transfers and allows efficient luminescence. These
two effects combined together contribute to the abnormal AIE phenomenon. Crossing
a CI is an ultrafast and efficient way to convert electronic excitation into heat
(vibrations and rotations) and formation of new chemical bonds. A simple alteration of
condition, e.g. a liquid/sold phase transition in AIE, can turn it on or off and lead to
dramatic changes in luminescence. Turning on/off the access to CI crossing is expected
to be of practical importance for tuning the properties of chromophores in different
phases in many applications, e.g. sensing or displays, with various external stimuli (not
limited only to phase transitions).
Methods
Molecular synthesis
The details of molecular synthesis are provided in the Supplementary Information.
Their UV-Vis absorption and photoluminescence spectra are provided in fig.S1&2.
UV-Vis pump/IR probe measurements
The details of ultrafast measurements are provided in SI. Ultrafast spectra and
dynamics of solid samples are provided in fig.s3&4 in SI, in addition to those in the
main text.
GC-MS characterization
Sample/THF (50 mM) solutions, with and without 18h-UV (254 nm) radiation
respectively, were used as the samples. Gas chromatography-mass spectrometer (GC-
MS) analyses were carried out on a gas chromatography (GC, Thermo, Trace1310)
system coupled to a electrostatic orbitrap mass spectrometer (Thermo, Q Exactive-GC)
equipped with an electrospray ionization (EI) source by using an TG-5HT column (30
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m, 0.25 mm, 0.25 μm). The analytes were eluted with a He gas with a flow rate of
1.0 mL/min.
Theoretical calculations
The geometries and vibrational frequencies of the intermediate structures of TPE,
molecule 2 and 3 were optimized and computed with the M06-2X[27] method and 6-
311++G(d,p) basis set. The SMD solvation model[28] was used for THF solvent. All the
calculations were performed with Gaussian 09 software[29]. Scaling factor of 0.952 was
used to improve description of spectra. The computed vibrational frequencies of the
cyclic intermediates are shown in fig. s10, which are consistent with our experimental
results.
Ground and excited energy profiles of TPE were computed with MP2 and ADC(2)
methods, respectively, using Turbomole 7.1.1 software.[30] Def2-SVP basis set along
with resolution of identity approximation were employed. Minimum energy crossing
points between ground and excited states were optimized using CIOpt code[31] coupled
with Turbomole. Although MP2 and ADC2 do not provide the correct dimensionality
of conical intersection, they typically provide good structures and energies and they are
recommended for larger molecular systems.[32] Optimized structures compare very well
to those obtained with multi-reference calculations (fig. S13). Nevertheless, we do not
intend to provide benchmark-level potential energy surfaces but rather a qualitative
picture to interpret experimental results. Structures in between optimized points in
fig.2f were interpolated using linear interpolation of internal coordinates. The barrier
on the photoisomerization pathway was not optimized, but represents the upper limit of
the true barrier height. Distances between optimized structures on reaction coordinate
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axis (x-axis) were scaled based on mean square displacements between the structures.
Supplementary information is available in the online version of the paper.
Acknowledgements
This material is based upon work supported by the National Science Foundation
of China (NSFC-21627805, 21673004, 21821004, 21674001 and 21790363) and
MOST (2017YFA0204702) China. We also acknowledge the support from the High-
performance Computing Platform of Peking University for the computational resources.
A.P, K.H.L. and C.C. acknowledge EPFL for funding. Insightful discussion with Prof.
Keith Nelson is deeply appreciated.
Author contributions
J.Z., D.Z. and J.G. designed experiments. J.Z. supervised the project. J.G., J.P.,
Z.Y. and J.Z. performed ultrafast experiments. R.W., H.H., J.L. and D.Z. prepared
materials. J.G., J.P. and J.Z. analyzed data. J.G., C.C., K.H.L. and A.P. conducted
theoretical calculations. J.Z. and J.G., A.P., C. C. and D.Z. prepared and revised the
manuscript.
Keywords: ultrafast UV/IR spectroscopy• aggregation-induced emission mechanism•
conical intersection
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